Motor System Lesions - Control of Posture and Movement

MOTOR SYSTEM LESIONS

Cortex and Corticospinal (Pyramidal) Lesions

Lesions of the cortex or corticospinal tract (e.g., stroke) result in muscle weakness without atrophy. Atrophy may ensue later because of disuse. The weakness is more in the extensors than the flexors in the upper limb, and so the upper limbs tend to be flexed. In the lower limb, it is the opposite, with the extensors being stronger. Stretch reflexes like the knee jerk tend to be brisk. There is dorsiflexion of the foot if the plantar response is elicited (positive Babinski’s sign). Because the tracts cross over at themedulla, the opposite half of the body is affected— hemiplegia. If the lesion is in the brainstem area,the functioning of cranial nerves that arise from there is also affected.

Spinal Cord Lesions

Spinal Shock

As soon as the spinal cord is injured or cut, it is fol-lowed by a period of spinal shock when all spinal reflex responses are depressed. This lasts for about two weeks in humans. The cause of spinal shock is uncertain.

With time, the spinal reflexes below the cut be-come exaggerated and hyperactive. It could be a re-sult of many reasons. One reason is the removal of the inhibitory effects of the higher motor centers. Also, the neurons become hypersensitive to the exci-tatory neurotransmitters. In addition, the spinal neu-rons may sprout collaterals that synapse with excita-tory input. Whatever the reason, the stretch reflexes are exaggerated and muscle tone increases. The first reflex response that comes back is a slight contrac-tion of the leg flexors and adductors in response to some painful stimuli.

The extent of disability depends on the level of the spinal cord that has been injured. It must be remem-bered that although the spinal cord has all the seg-ments—8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal—the length of spinal cord is shorter than the vertebral column and ends at level L1 and L2. Hence, injury below the second lumbar vertebra may affect only the muscles and dermatomes inner-vated by the sacral and coccygeal nerves.

If spinal cord injury occurs above the third cervi-cal spinal segment, other than the loss of voluntary movements of all the limbs, respiratory movements are affected as the phrenic nerve arising from C3, 4, 5 supplies the diaphragm. Loss of movement of all four limbs is known as quadriplegia. If the lesion is lower, only the lower limbs are affected, and this is termed paraplegia. If the nerves to only one limb are affected, it is referred to as monoplegia.

Other Complications of Spinal Cord Injuries

One common complication among people with spinal cord injuries is decubitus ulcer. Because vol-untary weight shifting does not occur, the weight of the body compresses the circulation to the skin over bony prominences, producing ulcers. These ulcers heal slowly and are prone to infection.

As a result of disuse, calcium from bones are reab-sorbed and excreted in the urine. This increases the incidence of calcium stones forming in the urinary tract. Paralysis of the muscles of the urinary bladder, in addition to stone formation, result in stagnation of urine and urinary tract infection.

When the spinal reflexes return, they are exagger-ated. For example, in a person with quadriplegia, the slightest of stimuli can trigger the withdrawal reflex and the stimulated limb flexes with flexion/extension responses from the other three limbs. With time, as a result of prolonged and repeated flexion, scar tissue may form in the limb and the limb becomes fixed in one position, known as contractures.

The function of the autonomic system below the level of lesion is also affected. Voluntary control is lost if the lesion is above the sacral segments, and reflex contractions of a bladder and rectum occur as soon as they get full. Bouts of sweating and blanching of the skin as a result of vasoconstriction of blood vessels may occur. Wide swings in blood pressure can occur as a result of imprecise blood pressure regulation.

Even though sexual reflexes are complex, with inte-gration at various levels, manipulation of the genitals in males can produce erection and even ejaculation.

Mass Reflex

Below the level of the injury, afferent stimuli can travel from one level to the other and even a slight stimulus to the skin can trigger many reflexes, such as emptying the bladder and rectum, sweating, and blood pressure changes. This is known as the massreflex. People with chronic spinal injuries use this re-flex to give them some degree of control over urina-tion and defecation. They can be trained to initiate these reflexes by stroking or pinching the thigh trig-gering the mass reflex intentionally.